1. Field of the Invention
The present invention relates to a switching power supply unit having a main switching element and a single or a plurality of auxiliary switching elements which perform ON/OFF operations in synchronism with or opposite to ON/OFF operations of the main switching element.
2. Description of the Related Art
In general, in electronic equipment such as electronic calculators and communication devices, a switching power supply unit has been widely used for supplying a stabilized DC voltage from a commercial AC power supply. As the switching power supply unit, there are provided switching power supply units having different circuit systems, such as a forward converter, or a fly-back converter, in which a main switching element, which is connected in series to a primary winding of a transformer, is repeatedly turned on/off to intermittently apply an input voltage to the transformer so as to obtain a DC output through a rectifying and smoothing circuit connected to a secondary winding of the transformer. In contrast with these switching power supply units, there are provided various kinds of switching power supply units, in which improvement in circuit characteristics can be achieved by adding a circuit which includes an auxiliary switching element for performing ON/OFF operations in synchronism with ON/OFF operations of a main switching element or in reverse thereto.
A description will be given of a structure of a conventional switching power supply unit having such an auxiliary switching element referring to the drawings.
First, the conventional art disclosed in Japanese Unexamined Patent Publication No. 8-317647 will be illustrated with reference to FIG. 13.
In this figure, reference numeral 50 represents a switching power supply unit, which has a partial-resonance converter circuit 51 and a drive circuit 52. Of these components, the partial-resonance converter circuit 51 is composed of capacitors C51, C52, C53, and C54, diodes D51, D52, and D53, a transformer T51, a main switching element S51, and an auxiliary switching element S52.
In addition, the drive circuit 52 is composed of an output control circuit 53, comparators 54 and 55, an inverter 56, an isolation circuit 57, a triangle-wave oscillator 58, a light-emitting element Pa of a photo coupler, a light-receiving element Pb of the photo coupler, a transistor Q51, and resistors R51, R52, and R53.
In the switching power supply unit having the above-described structure, the auxiliary switching element S52 performs ON/OFF operations opposite to the ON/OFF operations of the main switching element S51.
Next, another conventional art disclosed in Japanese Unexamined Patent Application No. 8-37777 will be illustrated with reference to FIG. 14.
In this figure, reference numeral 60 represents a switching power supply unit, in which an FET disposed on the secondary side of a transformer is used for rectification. This is generally referred to as a synchronous rectification system. The switching power supply unit 60 is composed of a transformer T61, an input capacitor C61, an FET Q61 as a main switching element, an FET Q62 as an auxiliary switching element, similarly, an FET Q63 as another auxiliary switching element, a choke coil L61, an output capacitor C62, a light-emitting-side photo coupler PA, a light-receiving-side photo coupler PB, comparators 61, 62, and 63, a triangle-wave oscillator 64, isolation an inverter 67, a control circuit 68, and a control-signal output circuit 69. Of these constituent parts, the control-signal output circuit 69 is composed of transistors Q64 and Q65, and resistors R61 to R65.
In the switching power supply unit 60 having the above-described structure, the FET Q62 performs ON/OFF operations in synchronism with the ON/OFF operations of the FET Q61, whereas the FET Q63 performs ON/OFF operations in opposition to the ON/OFF operations of the FET Q61.
In each of the above-described switching power supply units, however, the circuit part driving the auxiliary switching element is formed of an IC. Additionally, since a ground level is different between the main switching element and the auxiliary switching element, installation of an isolation circuit composed of photoelectric elements and/or transformers such as a pulse transformer and a photo coupler is necessary. In this way, when an IC, a pulse transformer, or the like is used, the circuit structure is complicated and production cost is increased. Further, and possibly more important such an increase in the number of parts can also increase in size and weight.